The rete mirabile, a blood capillary organ in the eel, will be used to quantitate and characterize vesicular transport of labeled solutes by capillary endothelium. EM tracer data indicates that permeability in the perfuse organ reflects transendothelial transport via the endothelial vesicular system and not between endothelial cells. Comparison of differential transport in the intact, perfused rete with capillaries isolated from the rete indicate that the same mechanism of solute discrimination functions in both systems and that it is exclusively an endothelial cell phenomenon. Physiological, cellular and ultrastructural data from the rete can thus be integrated into an unambiguous model of capillary permeability. Further investigations of capillary transport by vesicles are as follows: Physiological determinants of capillary transport in the rete will be determined including temperature, hydrostatic and osmotic pressure gradients, polarity and parallel vs antiparallel perfusion. Molecular determinants of differential transPort in intact and isolated rete caPillaries will bs investigated. Labeled solutes which vary in molecular size, charge, glucosylation and adsorptive and receptor membrane binding Properties will be compared with sucrose. an indicator of bulk fluid flow through the vesicular system. Isolates of continuous arterial and fenestrated venous caPillaries from the rete will be compared according to their differential transport of solutes. enzyme specificity and receptor properties. The effect of method of fixation (rapid freezing vs chemical fixation) on the relative numbers of endothelial vesicles labeled with electron dense tracers will be determined. Vesicle membranes will be isolated from rete caPillary endothelial cells and analyzed for specific lipid. protein and recePtor Properties. Data from these various experimental approaches will be incorporated into a comPrehensive theory of vesicle activity and function in capillary endothelial cells. The role that endothelial vesicles play in differential capillary permeability may have global influences in the maintenance of tissue homeostasis, specific organ function and conditions leading to pathogenic states. Elucidation of vesicle function will enhance understanding of the regulatory influences of the capillary endothelial cell in health and disease.